A prevailing mobile communications system such as a mobile telephony system divides its entire service area into rather small radio zones called cells to provide services. As shown in FIG. 1, such a system comprises a plurality of base stations 111-1-111-5 that cover the radio zones, and mobile stations 112-1-112-3 that communicates with the base stations 111-1-111-5 by establishing radio channels with them.
As main access methods between the base stations and mobile stations, there are frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In any access schemes, each base station transmits a channel for enabling the mobile stations to identify the base station to be connected. Such a channel is called a perch channel, here. In the FDMA scheme or TDMA scheme, the perch channels of the base stations utilize different radio frequencies so that the mobile stations can identify the base stations by the radio frequencies. On the other hand, the direct sequence (DS)-CDMA is a scheme that allows a plurality of users to communicate in the same radio frequency band by transmitting information by carrying out secondary modulation that spreads a conventional information data modulation signal using a high rate spreading code. The radio wave of each user is identified by the spreading code assigned to the user.
Incidentally, the radio signal transmitted from a base station at certain transmission power travels through space with a certain attenuation, and arrives at a receiving site. Since the attenuation the radio signal undergoes increases with the distance from the transmitting site to the receiving site, it is natural that a perch channel transmitted from a distant base station is received at a lower received level, and a perch channel transmitted from a near base station is received at a higher received level. In practice, however, the propagation loss is not determined only by the distance, but varies because of such conditions as geography and buildings. As a result, the received power of the perch channels from the base stations fluctuate sharply with the move of the mobile station. To receive the signals from the base stations with better quality, it is important for the mobile station to continuously monitor the perches from the base stations, and to select the best base station. In the condition in which the received levels of the perch channels from the base stations fluctuate sharply, perches above a required received level are switched incessantly because the received level of the current perch drops suddenly, or the received level of a perch unreceivable before increases abruptly above the receivable level. To enable the mobile station to trace the best perch channel even in such a condition, a common mobile communications system employs a method that sends information about perch channels used by neighboring base stations from the network side to the mobile station. The information about the neighboring base stations consists of the radio frequencies of perches in FDMA or TDMA system, and of spreading codes or information about their phases in a CDMA system. In such a method, it is necessary for system operators to prepare the information on the neighboring base stations so that the base stations can store the information in advance. The information about the neighboring base stations is decided manually using distances on a map, or using software that simulates actual propagation environment according to data on geography and buildings.
The conventional method, however, has a problem in that the neighboring base station information prepared in advance is inappropriate, it is impossible to quickly follow actual changes of the buildings or geography and the like because it is difficult for the conventional method to accurately estimate actual propagation environment. Inappropriate neighboring base station information presents a serious problem unfavorable to the mobile communication services such as bringing about disconnection of a call or unreceivable state of an incoming call, because it hinders handover control in a communication mode or a cell transition processing in a standby mode of the mobile station.
Direct Sequence CDMA (DS-CDMA) is a scheme for a plurality of users to carry out communications using the same radio frequency band by transmitting information through secondary modulation that spreads a conventional information data modulation signal with a high rate spreading code. Each user is identified by a spreading code assigned to the user. Therefore, a receiver must convert its wideband received input signal to the original narrow-band signal by a process called despreading before it carries out ordinary demodulation. In the course of the despreading, the receiver carries out correlation detection between the received signal and a spreading code replica synchronized to the spreading code phase of the received signal. In particular, synchronization between the spreading code replica of the receiver and the spreading code phase of the received signal at the start of communication is called “initial acquisition”.
A normal initial acquisition scheme of a spreading code is performed by multiplying the received signal by the spreading code replica on the receiving side, by calculating correlation between the two signals by integrating the product over a particular interval, and by carrying out square-law detection, followed by making a decision as to whether the synchronization is established depending on whether the output exceeds a threshold value. To detect the correlation, there are two methods: one uses a sliding correlator that carries out a time integral; and the other uses a matched filter that carries out a spatial integral. The matched filter is configured by placing a plurality of sliding correlators in parallel, and hence it can calculate correlation values over multiple tips by varying the spreading code replicas of respective taps in accordance with the spreading code. Accordingly, it is much faster than the sliding correlator, although its circuit scale and current consumption are greater than those of the sliding correlator.
The research-and-development and standardization have been underway of a wideband DS-CDMA system (abbreviated to W-CDMA system from now on) with a spread bandwidth of more than 5 MHz as a candidate of a next generation mobile communications system called IMT-2000. The W-CDMA system is an asynchronous system in which base stations each operate on independent time bases.
FIG. 13 illustrates spreading code assignment schemes in a down-link in an inter-base station asynchronous system and a synchronous system (“SF” in FIG. 13 is an abbreviation of a spreading factor). The cdma 2000 system or IS-95 proposed in the United States as a candidate of the IMT-2000 just as the W-CDMA, implements the inter-base station synchronization using the GPS.
Thus, the inter-base station synchronous system has a common time base for all the base stations, and hence the base stations can use the same spreading code with providing different delays for individual base stations. Accordingly, it is enough for the initial acquisition of the inter-base station synchronous system to only establish timing synchronization of the spreading code. On the other hand, since the inter-base station asynchronous system has no common time base for the base stations, the base stations employ different long codes (called scrambling codes because they convert signals from other cells into noise) to identify themselves. When power is turned on, a mobile station must establish the long code synchronization of a downlink common control channel from a cell site to connect itself to a base station (cell site) that provides the greatest power of the received signal. This operation is referred to as “cell search” in the sense that a cell site to which a radio channel is to be connected is searched for. In the inter-base station asynchronous system, the mobile station must carry out the cell search of all the long codes assigned to the system. In contrast, as for the inter-base station synchronous system, since the long code is limited to one type, it will be obvious that it can perform cell search, that is, establish long code synchronization of the downlink common control channel in a much shorter period than the asynchronous system.
The spreading code synchronization scheme disclosed in international publication No. WO97/33400 and Japanese patent application laid-open No. 11-196460 assigned to the assignee of the present application can speed up the cell search (long code synchronization of the downlink common control channel) of the inter-base station asynchronous system comparable to that of the inter-base station synchronous system. Specifically, the spreading code synchronization scheme takes the following steps as illustrated in FIG. 14. A perch channel (a channel to which the mobile station connects a radio link at the beginning of communication) is doubly spread by a short code that has the same cycle period as the symbol period and is used in common by all the base stations, and by a long code different from base station to base station; the long code spreading is masked at fixed intervals (in other words, the masked portions do not undergo the long code spreading) so that portions spread by only the short code are generated (a symbol spread by only the common short code is called a masked symbol from now on). Since the short code is common to all the base stations, the mobile station calculates the correlation between the received signal and the common short code that is used as the spreading code replica with the matched filter so that a peak is detected at the received timing of the short code spreading portions of the received signal independently of the type of the long codes. Storing the time of the correlation peaks in accordance with the timing of the masked symbols make it possible to establish the timing synchronization of the long code. After that, it is enough for the mobile station to identify the type of the long code that spreads the received signal, which is carried out by detecting the correlation at the timing that has already been obtained by using the spreading code generated by multiplying the short code by the long code as the replica code, and by making a threshold value decision. This fast 3-step cell search method using the long code mask can implement a fast cell search even in the inter-base station asynchronous system.
As for the standardization of the third generation mobile communications system, IMT-2000 (International Mobile Telecommunication—2000), it is developed by 3GPP (Third Generation Partnership Project). Configurations of the channels transmitted from the base station for the mobile station to establish synchronization with the system operated in inter-base station asynchronization is described in detail in the standard “3G TS 25.211 V3.2.0” or “3G TS 25.213”. To increase versatility, some modification is made from “Specification of Air-Interface for 3G Mobile System Volume 3”. A PSCH (Primary Synchronization CHannel) and a SSCH (Secondary Synchronization CHannel) are standardized as independent physical channels, wherein the PSCH is a channel the mobile station searches at first when establishing synchronization with the system, whereas the SSCH is a channel used for identifying the group of the spreading codes, the long interval spreading codes different for individual base stations. Although such modification is made of the physical configuration, the procedure that the mobile station starts the synchronization to the system using the PSCH, then identifies the group of the long interval spreading code using the SSCH, and finally identifies the long interval spreading code and its timing, and the effect achieved by the process are the same. Although the instant specification refers to “Specification of Air-Interface for 3G Mobile System Volume 3” for convenience sake, application to the 3GPP standard and its effect is the same.
The above is the description of the cell search of the mobile station in the initial acquisition. In a cellular system, however, the cell site providing the mobile station with the greatest received power, that is, the cell site to which the radio link is to be connected, changes as the mobile station moves during the communication. To carry out switching between the cell sites (soft handover), the receiver must regularly measure the received levels of the neighboring cell sites of the current cell site by establishing spreading code synchronization of the downlink perch channels. In the cell search at the soft handover, since the mobile station is notified of the types of the long codes of the neighboring cell sites from the current cell site, the cell search time becomes shorter than that in the initial acquisition.
Furthermore, the mobile station regularly carries out the cell search in the standby mode to search for a cell site to be connected when starting a traffic channel. In this case, the mobile station is also notified of the types of the long codes of the neighboring cell sites from the cell site with which the mobile station communicated last before entering the standby mode via the control channel, the cell search time also becomes shorter than that in the initial acquisition.
Since the inter-base station synchronous system utilizes only a single type long code, the individual cell sites use the long code by shifting it by a fixed time (by a fixed number of chips of the long code). Therefore, in the inter-base station synchronous system, it is enough for the cell search in the standby mode or for the cell search in the handover mode to carry out the search in only a search window around the long code phase that is shifted by the fixed time (fixed number of chips) from the long code phase of the perch channel of the cell site with which the mobile station communicated last before entering the standby mode, or of the current cell site during the handover, which enables quick cell search. Here, the search window refers to a search range obtained when considering the propagation delay from the each cell site.
In the foregoing inter-base station asynchronous system, however, since the long code phase of the perch channel of the cell site with which the mobile station communicated last before entering the standby mode, or the long code phase of the perch channel of the current cell site in the soft handover mode has nothing to do with the long code phases of the neighboring cell sites, the 3-step cell search in the foregoing initial acquisition using the long code mask must be carried out basically, so that it takes rather longer cell search time than the synchronous system. Therefore, the inter-base station asynchronous system has a problem of increasing the consumption power of the mobile station than the synchronous system especially in the standby mode, because the mobile station must operate its demodulating circuit longer.